Both series are very good. In my opinion, these are the best fantasy books since the Lord of the Rings. So my advice would be, to read both series
Since you have to start with one, I just tell you a little bit about the books.
Martin is more easy to access. His prose is more fluent and in my opinion better then Eriksons. There is also less repetition and a more strict structure to his books. His books are more grounded than Eriksons, much less magic and no super fighters, powers etc. It is also much easier to understand, what is happening and how the world works. The first three books are fantastic. The 4th and 5th not so great. And the series is not finished yet (okay, now that there is the tv-show, he is bound to write faster, hopefully). If you liked Abercrombie, you will like Martin. I have a few problems with the First Law's world in the original series, so I would say, Martins world building is much better.
Erikson is a different kind of beast. The world building is the best I know. But you will never get a objective view of the world or its history. All you know is what characters say or think about it. And sometimes they lie (in case of speaking) or their views are colored by emotions or self-illusion (in case of beeing in the heads of them). There is also a louder voice of a "narrator" in it, if I could call it so. Much more philosophical and sociological thoughts more direct in it, than in Martins work.
Erikson is sometimes a bit too vulgar, which can be distracting. But you get used to it. A little bit strange are the shifts between realistic and uber-powered scenes.
The first book is okay, but not so original as the later books. Maybe starting with the second book is not so bad. I would advice against reading the Esslemont novels in between. It is not needed and they are of very varying quality. The Malazan Books of the fallen are completed, what is a big difference to Martins books. Erikson writes at the moment the second book of a new trilogy, set in the same universe (and the first book was brilliant). Eriksons views on the world, war, culture, history etc. are very interesting and inspiring. The Letheri-books are a very good and interesting view at our society and economy etc. I could go on and on, but i have to finish sometime
So which to read? I don't know, both are brilliant